This bug has been seen and apparently fixed in bedrock edition but I couldn't find anyone talking about it being in the Java edition. Turtles will swim to the edge of enclosures and not stop. It looks like they are trying to get to other water sources and are not paying attention to the wall in the way. All the turtles in question were also hatched on an accessible beach and they are not therefore trying to get home. Sorry if this is a duplicate but I couldn't find anyone mention it for Java edition, only Bedrock.
Similar to MCPE-35422
Please check if that's still an issue for you in the latest 1.14 development snapshot.